Cancer cells undergo dramatic changes in carbohydrate expression during the onset and progression of disease. As a result, there has been considerable interest in understanding why these changes occur, how they contribute to the disease, and exploiting these changes for clinical use. Carbohydrate binding antibodies and lectins play a critical role in this research for several reasons. First, antibodies and lectins are used extensively to detect and monitor changes in carbohydrate expression. In this capacity, they are used for basic research, as diagnostics, and as therapeutic agents. A comprehensive understanding of their specificity is critical for proper use and interpretation of results. Until recently, this information was not readily available. With the advent of carbohydrate microarray technology, or glycan arrays, we can now rapidly analyze binding of these proteins to a wide variety of carbohydrate antigens. In the past, we have screened a large set of commercially available lectin and antibody reagents used in the research community. We found that many of these reagents did not display the expected specificity. The information we provided regarding specificity will enable better use of these reagents and facilitate interpretation of results. Recently, we have been profiling selected antibodies and lectins in collaborations with researchers at the German Cancer Research Institute, Abeome Corp., and University of Washington-St. Louis. In each case, the antibodies and lectins selectively bind tumors and display anti-cancer activity. They appeared to bind glycans, but the identities of the carbohydrate ligand(s) were unknown. The array has been used to profile the binding properties of these proteins and identify their ligands. In addition, the array can be used to develop new antibodies and lectins for monitoring new carbohydrate antigens. The array has been instrumental in the development of monoclonal antibodies that bind the GalNAca1-3Gal antigen. Using these antibodies, we have found that this carbohydrate antigen has altered expression on squamous cell carcinomas of the cervix and that changes in expression correlate with prognosis.